Method for improvement of nickel-chromium and nickel-chromium-iron alloys



United States Patent Int. Cl. C21d 1/ 78; C23f 1/00 US. Cl. 148-13 Claims ABSTRACT OF THE DISCLOSURE Nickel-chromium and nickel-chromium-iron alloys for use in high temperature service, while in worked condi tion are annealed in air at 1000-l350 C., at least twice. After each anneal, the scale formed during annealing is removed. This treatment improves heat resistance and decreases spray effect.

The improvement of nickel-chromium and nickelchromium-iron alloys for heating conductors and heatresistant structural materials may be obtained, as known, by small additions of certain elements. This process has been known for a long time and is used technically throughout the world. The most effective additives are alkaline earths and rare earths, especially calcium and cerium in amounts of some tenths of a percent. However, alkali elements, thorium, zirconium and titanium have also been suggested. These elements are added to the liquid melt of nickel-chromium and nickel-chromium-iron alloys, and the finished ingots are further processed according to the usual processes in the metal and steel industry, warm and/or cold to their final dimensions. In this case, the annealing conditions of the final annealing are adjusted so that as fine-grained a structure as possible is formed. As long as bright annealing is not done, it is also common to clean off the scale due to the annealing in mineral acids, where the pickling is carried out only until the surface is free of scale. Such layers of scale have the disadvantage that they do not have sufficient adhesion under the stress of temperature changes, and the scale particles spray around. The phenomenon known as the spray effect can be very disturbing in later operations by discoloring, for example, the quartz tube in IR radiators and thus affect adversely the heat transmission or lead to a considerable difficulty in enameled ovens by the imposition of stains on enamels and glazes.

Surprisingly, it has now been shown that in the case of nickel-chromium and nickel-chromium-iron alloys, as they are used for heating conductors and heat-resistant structural materials, and as long as they contain additions of the above-mentioned type and amount, a further considerable increase in heat-resistance and a significant decrease or complete suppression of the spray effect is possible if the finished drawn, or rolled wires, strips or sheets are annealed to obtain a coarse-grained structure, one or more times, preferably two to five times for one minute to 24 hours at temperatures between 1,000 and 1,350 C., preferably in air, and that after each annealing the material is etched in inorganic acids, for example 15 to 20% HCl, until the annealing scale is completely removed, where a short time annealing will be applied at higher temperatures for example between 1,200 and 1,350 C. and a long time annealing of preferably one hour up to 24 hours will be carried out at lower temperatures such as from 1,000 up to 1,200 C. It has been proved to be of advantage to continue the etching after 3,434,891 Patented Mar. 25, 1969 each annealing for another 5 to 15 minutes after removal of the scale. With more advantage, the annealing treatment is carried out at temperatures which are higher by approximately 50 than the anticipated operating condition of the heating element or the heat-resistant structural material. Experiments with IR radiators have shown that Wires of nickel-chromium alloy with nickel and 20% chromium at the ordinary operating temperatures of 900 to 1,000 C. do not tend to spray when they have been pretreated according to the method of the invention. In this case, it appeared that a twice repeated annealing of 6 hours each at 1200 C. in air and each with its etching in 15 to 20% HCl at a temperature of 60-80" C., was completely sufficient. The examination was carried out in such a way that the quartz radiator was alternately turned on and off every 2 minutes. After 3,000 cycles, no trace of scale spraying could be observed in the tube, while the quartz tube in the case of a nontreated wire of the same composition was noticeably deformed by sprayed-off scale particles.

Thus, the invention provides a process for treating nickel-chromium or nickel-chromium-iron alloy which contains a known additive for improving heat-resistance and suppressing spray effect, in an amount suitable to that purpose. The alloy is treated when in warped condition, e.g. drawn or rolled into wires, strips, sheets, etc. The treatment involves annealing at least once for at least one minute at a temperature of about 1000-l350 C., and after each annealing, etching with inorganic acid until annealing scale is substantially completely removed. Best results are obtained when the annealing is conducted in air. Preferably the annealing is carried out about 2-5 times and the annealing temperature for each annealing is about one hour to 24 hours at about 1200-l350 C.

As mentioned above, the annealing temperature is preferably at least about 50 C. above the temperature to which the Worked piece will be subjected when used. Accordingly, the invention contemplates that the annealing be carried out at a temperature at least 50 higher than the rated temperature for the worked piece. The rated temperature is indicated in the conventional manner, for example by marking the element or the article in which it is included to indicate such temperature.

While the invention has been described with respect to particular embodiments thereof, these embodiments are merely representative of the invention and do not serve to define the limits thereof.

The following examples show the process of the invention:

Example I A wire of the following composition:

Cr 19.65 Si 0.96 Mn 0.85 Cu 0.10 C 0.03 Ce 0.21 Ca 0.08 Ni Balance is subjected to a twice repeated annealing treatment of three minutes each at a temperature of 1300 C. in air, is then etched for ten minutes in a 15% HCl at 60 C. after each of the above mentioned annealing treatments.

The increase of the heat-resistance is reached by increasing the adhesion of the scale. Thereby the spray effect is decreased and the scale adhering to the wire is a protective layer against continuous scaling. This avoids a continuous loss of metal.

3 Example II A container for workpieces to be enameled for use in an enameling oven, made of an alloy of the following composition:

Percent Ni 74.2 Cr 16.5 Fe 8.5

Ti 0.3 Si 0.25

encountered considerable difiiculties in enameling of steel sheets by spraying of scale from its surface during the heating and cooling and thus producing dark spots resulting in a considerable refuse. The container was then treated according to the process of the invention. The container was anealed twice for six hours each at a temperature of 1150 C. and was then removed from the oven and cooled after six hours and twelve hours with etching in HCl at a temperature of C. for a period of twenty minutes after each six hours annealing. After said treatment the container could be used satisfactorily for the above purpose without that said spots were formed on the enamel by spraying scale.

What is claimed is:

1. Process for treating nickel-chromium or nickel-chromium-iron alloy which contains as an additive an alkaline earth metal, alkali metal, rare earth, thorium, zirconium,

titanium or mixtures thereof in a minor proportion sufiicient to improve heat resistance and suppress spray effect, and is in Worked condition, which comprises:

(a) annealing in air at least twice for at least one minute at a temperature of about 10001350 C., and

(b) after each annealing, etching with inorganic acid until annealing scale is substantially completely removed.

2. Process according to claim 1, wherein the annealing step is carried out 2-5 times, and the annealing time for each annealing is about 1-24 hours at about 1200-1350 C. in air.

3. The process of claim 1 wherein the inorganic acid is 15-20% HCl.

4. The process of claim 1, wherein the etching following each annealing is continued for about 5-15 minutes after the scale is substantially completely removed.

5. The process of claim -1, wherein the alloy contains said additive in amount of a fraction of 1%.

'6. The process of claim 2, wherein the alloy contains said additive in amount of a fraction of 1%.

7. The process of claim 2, wherein the inorganic acid is 15-20% HCl, the etching following each annealing is continued for about 5-15 minutes after the scale is substantially completely removed, and wherein the alloy contains said additive in amount of a fraction of 1%.

'8. The process of producing an article having an element composed of nickel-chromium or nickel-chromiumiron alloy which contains as an additive an alkaline earth metal, alkali metal, rare earth, thorium, zirconium, titanium or mixtures thereof in a minor proportion sufficient to improve heat resistance and suppress spray effect, and is in worked condition, and is rated for a maximum service temperature, which comprises:

(a) annealing the element in said worked condition in air, at least twice, for at least one hour at a temperature of about 1000-1350 C. and at least 50" higher than said rated temperature;

(b) after each annealing, etching with inorganic acid until annealing scale is substantially completely removed.

9. Process according to claim 8, wherein the annealing step is carried out 2-5 times and the annealing time for each annealing is about 2-24 hours at about 1200-1300 C. in air.

10. The process of claim 9, wherein the inorganic acid is 15-20% HCl, the etching following each annealing is continued for about 5-15 minutes after the scale is substantially completely removed, and wherein the alloy contains said additive in amount of a fraction of 1% References Cited UNITED STATES PATENTS 1,970,268 8/1934 Bellis 14820.6 2,190,781 2/1940 Guertler 171 2,400,255 5/1946 Pfeil 75-171 2,432,149 12/1947 Griffiths 75171 2,793,948 5/1957 Wagner 148-11.5 X 2,913,360 11/1959 Zaremski et al 13441 X 3,093,476 6/1963 Gittus 75-171 OTHER REFERENCES Metals Handbook, 1948 Edition, published by ASM, relied on pp. 1031-1033 and 1040-1044.

CHARLES N. LOVELL, Primary Examiner.

US. Cl. X.R. 134-41; 148-63 

